The glutathione S-transferases (GST) are a ubiquitous family of enzymes with dual substrate specificities that perform important biochemical functions including xenobiotic biotransformation and detoxification, drug metabolism, and protection of tissues against peroxidative damage and subsequent inflammatory responses. The basic reaction catalyzed by these enzymes is the conjugation of an electrophilic substrate with reduced glutathione (GSH) and results in either activation or deactivation/detoxification of the substrate. The requirement for conjugating reduced GSH to a wide variety of substrates necessitates a diversity in GST structures in various organisms and cell types.
GSTs are homodimeric or heterodimeric proteins localized in the cell cytosol. The major isozymes share common structural and catalytic properties, and the human isozymes have been classified into four major classes, Alpha, Mu, Pi, and Theta. The two largest classes, Alpha and Mu, are identified by their respective protein isoelectric points: pI.about.7.5-9.0 (Alpha) and pI.about.6.6 (Mu). Each GST possesses a common binding site for GSH and a variable hydrophobic binding site. The hydrophobic binding site in each isozyme is specific for particular electrophilic substrates.
In most cases, GSTs perform the beneficial function of deactivating and detoxifying potentially mutagenic and carcinogenic chemicals. However, in some cases their action is detrimental and produces mutagenic and carcinogenic compounds. Some forms of rat and human GSTs are thus reliable preneoplastic markers. Expression of human GSTs in bacterial strains, such as Salmonella typhimurium, used in the well known Ames test for mutagenicity, has helped to establish the role of these enzymes in mutagenesis. Studies have shown that dihalomethanes are more mutagenic in bacterial cells which express human GST than in cells which do not express GST (Thier, R. et al. (1993) Proc. Natl. Acad. Sci. 90: 8576-8580). The mutagenicity of ethylene dibromide and ethylene dichloride is increased in bacterial cells expressing the human Alpha GST, A1-1, while the mutagenicity of aflatoxin B1 is substantially reduced by enhancing the expression of GST (Simula, T. P. et al. (1993) Carcinogenesis 14: 1371-6).
GST has been implicated in the acquired resistance of many cancers to drug treatment. Multi-drug resistance occurs when cancer cells are treated with and subsequently become resistant to cytotoxic drugs. In some drug resistant cancers, elevated GST levels are observed. It is believed that the drug being used to treat the cancer is deactivated by the GST-catalyzed GSH conjugation reaction. The increased GST levels also protect the cancer cells from other cytotoxic agents for which that GST has affinity. Increased levels of A1-1 in tumors have been linked to drug resistance induced by cyclophosphamide treatment (Dirven H. A. et al. (1994) Cancer Res. 54: 6215-20).
Leukotrienes are biologically active compounds that function as mediators of various inflammatory processes, such as leukocyte chemotaxis. 5-Lipoxygenase-activating protein (FLAP) and leukotriene C4 (LTC4) synthase, two proteins involved in leukotriene biosynthesis, are 31% identical at the amino acid level. 5-Lipoxygenase catalyzes a two-step reaction from arachidonic acid to the unstable epoxide LTA4 via 5-hydroxyperoxyeicosatetraenoic acid A novel member of the FLAP/LTC4 synthase genes are the microsomal glutathione-S-transferase II (microsomal GST II) family. FLAP increases the efficiency of 5-lipoxygenase conversion of 5-hydroxyperoxyeicosatetraenoic acid to LTA4. The epoxide LTA4 can be hydrolyzed to LTB4 by the cytosolic protein LTA4 hydrolase or conjugated with reduced glutathione by LTC4 synthase to LTC4. Increased levels of LTC4 occurs in leukocytes from patients with chronic myelogenous leukemia. Microsomal GST-II has 33% amino acid identity to FLAP, 44% amino acid identity to LTC4 synthase, and 11% amino acid identity to microsomal GST I. Microsomal GST II has a wide tissue distribution at the mRNA level and is expressed in human liver, spleen, skeletal muscle, heart, adrenals, pancreas, prostate, testis, fetal liver, and fetal spleen.
The discovery of a new microsomal glutathione-S-transferase and the polynucleotides encoding it satisfies a need in the art by providing new compositions which are useful in the diagnosis, prevention and treatment of diseases associated with cell proliferation, in particular, cancers and immune response.